1. Field of the Invention
The present invention relates to an exposure apparatus inspection method used in a semiconductor device manufacturing process and an exposure apparatus for facilitating the inspection method.
2. Description of the Related Art
One of the important matters to keep the yield of a semiconductor device high is to maintain an exposure apparatus used in a photolithographic step in a normal image formation state. To do so, a technique for inspecting and monitoring the state of the exposure apparatus by a simple method is required.
If the shape and brightness distribution of the secondary light source of the exposure apparatus change, the characteristic of the exposure apparatus related to the image formation of a mask pattern on a substrate surface changes. Accordingly, it is necessary to constantly inspect the illumination optical system of the exposure apparatus so as to keep the illumination optical system in the same state.
Factors for the error of the illumination optical system that may influence the image formation characteristic of the exposure apparatus include a phenomenon (illumination axis offset) that an illumination light incident on a photomask is inclined as a whole in addition to the shape and brightness distribution of the secondary light source.
The influence of the illumination axis offset will be explained with reference to FIGS. 19A and 19D.
In a state of no illumination axis offset (FIGS. 19A to 19B), there is no positional deviation between a resist pattern 103A which is obtained by exposure of a light 102A while a substrate (photosensitive substrate) 100 having a resist applied onto a wafer is located on an image plane 101 (in a best focus state) and resist patterns 103B, 103C which are obtained by exposure of lights 102B and 102C, respectively, in a state slightly defocused from the best focus. Due to this, the exposure apparatus in a state in which no illumination axis offset occurs is less influenced by the change of focus.
On the other hand, in a state of illumination axis offset (FIGS. 19C to 19D), the formation positions of resist patterns 103A to 103C are shifted laterally by the defocus of the photosensitive substrate 100. Due to this, a desired pattern may not be possibly formed in an entire exposure region. This causes an increase in the number of defects.
Such a disadvantage can be prevented by inspecting whether illumination axis offset occurs, and stopping the exposure apparatus to conduct maintenance on the system if it is determined that illumination axis offset occurs. The inspection of the shape, that of the brightness distribution, and that of the illumination axis offset of the secondary light source will be referred to as “the inspection of the illumination optical system” hereinafter.
To inspect a semiconductor device manufacturing apparatus, a method for simply measuring the apparatus in a short time and without stopping the apparatus is desirous. Illumination optical system inspection methods that satisfy this requirement are as follows.
An aperture pattern around which light is shield (pinhole pattern) is prepared, and the aperture pattern is located at a position non-conjugate with the surface of the photosensitive substrate. Using the aperture pattern thus located, exposure is performed. As a result of the exposure, an image of the secondary light source is formed on the photosensitive substrate. Based on the secondary light source image, the shape of the secondary light source or brightness distribution thereof is measured (U.S. Pat. No. 5,973,771, Proceedings of SPIE vol. 3334, pp. 281-288).
According to this method, the aperture pattern is made to function as a lens of a pinhole camera to thereby transfer the secondary light source image onto the photosensitive substrate and the inspection is conducted by observing the transferred image.
There is known another inspection method for acquiring a secondary light source transferred image using not a simple aperture pattern, but a zone plate (U.S. Pat. No. 6,048,651).
However, illumination axis offset cannot be recognized according to these two methods disclosed in the above-identified documents for the following reasons.
A pattern formed on the photosensitive substrate is an image that represents the brightness distribution of the secondary light source. The image contains no information on a projection optical system. Due to this, even if the transferred image is observed, it cannot be determined whether the verticality of an illumination light relative to the projection optical system is maintained.
As a method for measuring the shape or brightness distribution of the secondary light source and, at the same time, measuring an illumination axis offset, there is known a measurement method using a pattern (grating pinhole pattern, see, for example, FIG. 20) having a diffraction grating disposed inside of an aperture pattern around which light is shielded (U.S. Pat. No. 6,317,198). In this method, a diffracted light generated by the diffraction grating is used. Using the diffracted light, the position of a diaphragm that specifies the numerical aperture of the projection optical system is transferred onto the photosensitive substrate simultaneously with the brightness distribution of the secondary light source.
FIG. 21 illustrates one example of a pattern formed on the photosensitive substrate by the above-stated method. A 0th-order diffracted light image 110 contains information on the brightness distribution of the secondary light source. A 1st-order diffracted light image (profile image) 111 contains information on the position of the profile of the diaphragm. The positional deviation between the center of the 0th-order diffracted light image 110 and the center of the diaphragm profile image 111 represents the amount of illumination axis offset.
According to this method, however, the pitch of the diffraction grating on a photomask used in the inspection is required to be set quite small. This will be explained concretely as follows.
The pitch of the diffraction grating on the photomask used in the inspection of the illumination optical system has two restriction conditions related to a diffracted light. Namely, it is necessary to set the pitch of the diffraction grating so that the 0th-order diffracted light image is not overlapped with the 1st-order diffracted light image and so that the primary diffracted light passes through the diaphragm position (pupil end).
If the state of an exposure apparatus having an exposure wavelength of 193 nm and a projection reduction ratio of 1/4 in which the emission-side numerical aperture (NA) of a projection optical system is set at 0.68 and a coherence factor (σ) is set at 0.75 is inspected, the appropriate pitch of the diffraction grating necessary to satisfy the two conditions is approximately 0.64 μm. If this pitch is converted into a scale on the photosensitive substrate, it is a half pitch of 0.08 μm. This half pitch is smaller than the minimum half pitch of a repetition pattern of 0.13 to 0.11 μm used in currently mass-produced semiconductor devices.
To create a photomask having such a fine pattern, an advanced technique is required, which disadvantageously pushes up cost. Thus, it is difficult to inspect the illumination axis offset of the exposure apparatus at as low a cost as in the prior art.